Nitride fluorescent material, method for producing the same, and light emitting device

ABSTRACT

A method for producing a nitride fluorescent material having high emission luminance can be provided. The method includes heat-treating a raw material mixture containing silicon nitride, silicon, an aluminium compound, a calcium compound, and a europium compound.

CROSS-REFERENCE TO RELATED APPLICATION

This is a Divisional of U.S. application Ser. No. 15/247,083 filed Aug.25, 2016, which claims priority to Japanese Patent Application No.2015-169327, filed on Aug. 28, 2015 and Japanese Patent Application No.2016-141227, filed on Jul. 19, 2016, the entire disclosures of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to a nitride fluorescent material, amethod for producing the nitride fluorescent material, and a lightemitting device.

Description of the Related Art

Light emitting devices have been developed that emit white light byincorporating in combination a blue light emitting LED (Light EmittingDiode) as a light emitting element, a fluorescent material emittinggreen light when excited by the blue light, and a fluorescent materialemitting red light when excited by the blue light. For instance,Japanese patent application JP 2008-303331 A describes a light emittingdevice that emits white light by incorporating in combination a β sialonfluorescent material having a β-type Si₃N₄ crystalline structure, andemitting green light; a nitride fluorescent material having acomposition of CaAlSiN₃:Eu and emitting red light (hereinafter alsoreferred to as “CASN fluorescent material”); and a blue LED.

A red light emitting fluorescent material (hereinafter also referred toas “SCASN fluorescent material”) having a composition of(Ca,Sr)AlSiN₃:Eu obtained by partially replacing Ca with Sr in a CASNfluorescent material is known. A SCASN fluorescent material is said tohave a peak emission wavelength shorter than the peak emissionwavelength of a CASN fluorescent material. A CASN fluorescent materialcan be obtained by, for instance, calcinating a mixture comprisingsilicon nitride, aluminium nitride, calcium nitride, and europiumnitride. A SCASN fluorescent material can also be obtained in the samemanner as a CASN fluorescent material (see, for instance, JP 2006-8721A).

SUMMARY OF INVENTION

A method for producing a nitride fluorescent material having highemission luminance can be provided. The method includes heat-treating araw material mixture containing silicon nitride, elemental silicon, analuminium compound, a calcium compound, and a europium compound.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing an example of a lightemitting device.

FIG. 2 is an example of a light emission spectrum of relative energyversus wavelength for a nitride fluorescent material according to thepresent embodiment.

FIG. 3 is a scanning electron microscope (SEM) image of a nitridefluorescent material according to Comparative Example 1.

FIG. 4 is an SEM image of a nitride fluorescent material according toExample 1.

FIG. 5 is an example of light emission spectrum of relative energyversus wavelength for a nitride fluorescent material according to thepresent embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENT

A method for producing a nitride fluorescent material of the presentinvention will be described below with reference to embodiments. Theembodiments shown below, however, exemplify the technical concept of thepresent invention, and the present invention is not limited to thefollowing method for producing a nitride fluorescent material. Therelationship between the color names and the chromaticity coordinates,the relationship between the wavelength ranges of light and the colornames of monochromatic light, and others are in accordance with JapaneseIndustrial Standard (JIS) Z8110. As used herein, the term “step” meansnot only an independent step but also a step which cannot be clearlydistinguished from the other steps but can achieve the anticipatedeffect of that step. Further, for the amount of each component containedin a composition, when a plurality of substances corresponding to thecomponent exist, the amount of the component means the total amount ofthe substances present in the composition unless otherwise specified.

[Method for Producing a Nitride Fluorescent Material]

The method for producing a nitride fluorescent material includesheat-treating a raw material mixture containing silicon nitride,elemental silicon, an aluminium compound, a calcium compound, and aeuropium compound. The nitride fluorescent material has a compositionrepresented by, for instance, formula (I).

Sr_(s)Ca_(t)Al_(u)Si_(v)N_(w):Eu  (I)

In the formula (I), s, t, u, v, and w respectively satisfy 0.0≤s<1,0<t≤1, s+t≤1, 0.9≤u≤1.1, 0.9≤v≤1.1, and 2.5≤w≤3.5.

The raw material mixture contains elemental silicon as well as siliconnitride as a silicon source. Although the details are unknown, when heattreated, elemental silicon is believed to react while undergoingnitridization. Because of this, sintering due to heat treatment at ahigh temperature is believed less likely to occur. Thus, a nitridefluorescent material of a large particle diameter can be obtained. Theresultant nitride fluorescent material has high emitting efficiency andimproved emitting luminance.

The raw material mixture contains silicon nitride, elemental silicon, atleast one aluminium compound, and at least one europium compound.

The silicon nitride is a silicon compound containing a nitrogen atom andsilicon atom, and may contain an oxygen atom. When the silicon nitridecontains an oxygen atom, the oxygen atom may be contained in the form ofa silicon oxide or in the form of an oxynitride of silicon.

The content of the oxygen atom in the silicon nitride may be, forinstance, less than 2% by weight, or 1.5% by weight or less. The contentof the oxygen atom may be also, for instance, 0.3% by weight or more, or0.4% by weight or more. When the amount of oxygen is equal to or greaterthan a predetermined value, the reactivity may be increased, and theparticle growth may be promoted. When the amount of oxygen is equal toor less than a predetermined amount, excessive sintering of thefluorescent material particles may be suppressed, improving the shapesof the fluorescent material particles to be produced.

The purity of the silicon nitride may be, for instance, 95% by weight ormore, or 99% by weight or more. When the purity of the silicon nitrideis equal to or greater than a predetermined value, the influence ofimpurities may be minimized, leading to further improved emittingluminance of the nitride fluorescent material to be produced.

The average particle diameter of the silicon nitride may be, forinstance, from 0.1 μm to 15 μm, or from 0.1 μm to 5 μm. When the averageparticle diameter of the silicon nitride is equal to or less than apredetermined value, the reactivity during production of the nitridefluorescent material may be improved. When the average particle diameterof the silicon nitride is equal to or greater than a predeterminedvalue, excessive reaction during production of the nitride fluorescentmaterial may be suppressed, which suppresses sintering of thefluorescent material particles to be produced.

The silicon nitride may be appropriately selected from commerciallyavailable products, or may be produced by nitriding silicon. The siliconnitride can be obtained, for instance, by grinding silicon to be used asa raw material in an inert gas atmosphere such as rare gas and nitrogengas, and heat-treating the resultant powder in a nitrogen atmosphere sothat the powder is nitrided. The elemental silicon to be used as the rawmaterial preferably has high purity, and the purity may be, forinstance, 3 N (99.9% by weight) or more. The average particle diameterof the ground silicon may be, for instance, from 0.1 μm to 15 μm. Theheat-treating temperature may be, for instance, from 800° C. to 2000°C., and the time for heat-treating may be, for instance, from 1 hour to20 hours. The resultant silicon nitride may undergo, for instance,grinding treatment in a nitrogen atmosphere.

The silicon as a component of the raw material mixture is elementalsilicon. The purity of the silicon may be, for instance, 95% by weightor more, or 99.9% by weight or more. When the purity of the silicon isequal to or greater than a predetermined value, the influence ofimpurities may be minimized, leading to further improved luminance ofthe fluorescent material to be produced.

The average particle diameter of the silicon may be, for instance, from0.1 μm to 100 μm, or from 0.1 μm to 80 μm. When the average particlediameter of the silicon nitride is equal to or less than a predeterminedvalue, complete nitriding into the inside of the particles may beachieved. When the average particle diameter of the silicon is equal toor greater than a predetermined value, excessive reaction duringproduction of the nitride fluorescent material may be suppressed, andthus sintering of the fluorescent material particles may be suppressed.

A portion of silicon nitride and elemental silicon in the raw materialmixture may be replaced with another silicon compound, such as siliconoxide. That is, the raw material mixture may contain a silicon compoundsuch as silicon oxide in addition to silicon nitride and elementalsilicon. Examples of the silicon compound include silicon oxide, siliconoxynitride, and silicate.

A portion of silicon nitride and elemental silicon in the raw materialmixture may be replaced with a metal compound, a simple metal, or analloy of elements from Group IV of the periodic table, such asgermanium, tin, titanium, zirconium, and hafnium. Examples of the metalcompound include oxide, hydroxide, nitride, oxynitride, fluoride, andchloride.

The weight percentage of the silicon relative to the total amount of thesilicon nitride and silicon in the raw material mixture may be, forinstance, from 10% by weight to 85% by weight, from 20% by weight to 80%by weight, or from 30% by weight to 80% by weight. When the weightpercentage of the silicon is equal to or greater than a predeterminedvalue, sintering of the particles during particle growth of the nitridefluorescent material may be suppressed. Furthermore, since siliconnitride has an effect of accelerating nitriding reaction of silicon,incorporating a predetermined weight percentage or less of silicon (tohave a greater weight percentage of the silicon nitride) leads tosufficient nitriding of silicon.

Examples of the aluminium compound include an aluminium-containingoxide, hydroxide, nitride, oxynitride, fluoride, and chloride. In placeof at least a portion of the aluminium compound, for instance, simplealuminium metal or an aluminium alloy may be used. Specific examples ofthe aluminium compound include aluminium nitride (AlN), aluminium oxide(Al₂O₃), and aluminium hydroxide (Al(OH)₃), and it is preferable to useat least one selected from the group consisting of these, and it is morepreferable to use aluminium nitride. Since aluminium nitride is composedonly of the elements to be contained in the target fluorescent materialcomposition, introduction of impurities can be prevented moreeffectively. Compared to, for instance, an aluminium compound containingoxygen or hydrogen, aluminium nitride can reduce the influence of theseelements, and needs no nitriding reaction unlike simple metals. Thesealuminium compounds may be used individually, or two or more of them maybe used in combination.

The average particle diameter of the aluminium compound to be used as araw material may be, for instance, from 0.1 μm to 15 μm, or from 0.1 μmto 10 μm. When the average particle diameter of the aluminium compoundis equal to or less than a predetermined value, the reactivity duringproduction of the nitride fluorescent material may be improved. When theaverage particle diameter of the aluminium compound is equal to orgreater than a predetermined value, sintering of the fluorescentmaterial particles during production of the nitride fluorescent materialmay be suppressed.

The purity of the aluminium compound may be, for instance, 95% by weightor more, or 99% by weight or more. When the purity of the aluminiumcompound is equal to or greater than a predetermined value, theinfluence of impurities may be minimized, leading to further improvedlight emitting luminance of the fluorescent material.

The aluminium compound may be appropriately selected from commerciallyavailable products, or a desired aluminium compound may be produced. Forinstance, the aluminium nitride may be produced by, for example, thedirect nitriding method of aluminium.

At least a portion of the aluminium compound in the raw material mixturemay be replaced with a metal compound, a simple metal, or an alloy ofelements from Group III of the periodic table, such as gallium, indium,vanadium, chrome, and cobalt. Examples of the metal compound includeoxide, hydroxide, nitride, oxynitride, fluoride, and chloride.

Examples of the calcium compound may include calcium-containing hydrogenoxide, oxide, hydroxide, nitride, oxynitride, fluoride, and chloride. Inplace of at least a portion of the calcium compound, for instance, asimple calcium metal or an alloy of calcium may be used. Specificexamples of the calcium compound include inorganic compounds such ascalcium hydride (CaH₂), calcium nitride (Ca₃N₂), calcium oxide (CaO),calcium hydroxide (Ca(OH)₂), and salts of organic compounds such as animide compound and an amide compound. It is preferable to use at leastone selected from the group consisting of these, and calcium nitride ismore preferable. Since calcium nitride is composed only of the elementsto be contained in the composition of the target fluorescent material,introduction of impurities can be prevented more effectively. Comparedto, for instance, a calcium compound containing oxygen or hydrogen,calcium nitride can reduce the influence of these elements, and needs nonitriding reaction unlike simple metals. These calcium compounds may beused individually, or two or more of them may be used in combination.

The average particle diameter of the calcium compound to be used as araw material may be, for instance, from 0.1 μm to 100 μm, or from 0.1 μmto 80 μm. When the average particle diameter of the calcium compound isequal to or less than a predetermined value, the reactivity duringproduction of the nitride fluorescent material may be improved. When theaverage particle diameter of the calcium compound is equal to or greaterthan a predetermined value, sintering of the nitride fluorescentmaterial particles during production of the nitride fluorescent materialmay be suppressed.

The purity of the calcium compound may be, for instance, 95% by weightor more, or 99% by weight or more. When the purity is equal to orgreater than a predetermined value, the influence of impurities may beminimized, leading to further improved light emitting luminance of thefluorescent material.

The calcium compound may be appropriately selected from commerciallyavailable products, or a desired calcium compound may be produced. Forinstance, calcium nitride may be obtained by grinding calcium to be usedas a raw material in an inert gas atmosphere, and heat-treating theresultant powder in a nitrogen atmosphere so that the powder isnitrided. The calcium to be used as a raw material is preferably highlypure, and the purity may be, for instance, 2 N (99% by weight) or more.The average particle diameter of ground calcium may be, for instance,from 0.1 μm to 15 μm. The heat-treating temperature is, for instance,from 600° C. to 900° C., and the time for heat-treating may be, forinstance, from 1 hour to 20 hours. The resultant calcium nitride mayundergo, for instance, grinding treatment in an inert gas atmosphere.

At least a portion of calcium compound in the raw material mixture maybe replaced with a metal compound, a simple metal, an alloy, or the likeof alkaline earth metals such as magnesium and barium; alkali metalssuch as lithium, sodium, and potassium; and elements from Group III ofthe periodic table, such as boron and aluminium. Examples of the metalcompound include hydride, oxide, hydroxide, nitride, oxynitride,fluoride, and chloride.

Examples of the europium compounds include europium-containing oxide,hydroxide, nitride, oxynitride, fluoride, and chloride. In place of atleast a portion of the europium compound, for instance, a simpleeuropium metal or an alloy of europium may be used. Specific examples ofthe europium compound include europium oxide (Eu₂O₃), europium nitride(EuN), and europium fluoride (EuF₃). At least one selected from thegroup consisting of these is preferable, and europium oxide is morepreferable. Since europium nitride (EuN) is composed only of theelements to be contained in the target composition of the fluorescentmaterial, introduction of impurities can be more effectively prevented.Furthermore, since europium oxide (Eu₂O₃) and europium fluoride (EuF₃)also serve as a flux, these are preferably used. These europiumcompounds may be used individually, or two or more of them may be usedin combination.

The average particle diameter of the europium compound to be used as araw material may be, for instance, from 0.01 μm to 20 μm, or from 0.05μm to 10 μm. When the average particle diameter of the europium compoundis equal to or greater than a predetermined value, agglomeration of thefluorescent material particles during production may be suppressed. Whenthe average particle diameter of the europium compound is equal to orless than a predetermined value, more uniformly activated fluorescentmaterial particles can be obtained.

The purity of the europium compound may be, for instance, 95% by weightor more, or 99.5% by weight or more. When the purity of the europiumcompound is equal to or greater than a predetermined value, theinfluence of impurities may be minimized, leading to further improvedlight emitting luminance of the fluorescent material.

The europium compound may be appropriately selected from commerciallyavailable products, or a desired europium compound may be produced. Forinstance, the europium nitride may be produced by grinding europium tobe used as a raw material in an inert gas atmosphere, and heat-treatingthe resultant powder in a nitrogen atmosphere so that the powder isnitrided. The average particle diameter of the ground europium may be,for instance, from 0.1 μm to 10 μm. The heat-treating temperature maybe, for instance, from 600° C. to 1200° C., and the time forheat-treating may be, for instance, from 1 hour to 20 hours. Theresultant europium nitride may undergo, for instance, grinding treatmentin an inert gas atmosphere.

At least a portion of the europium compound in the raw material mixturemay be replaced with a metal compound, a simple metal, or an alloy ofrare-earth elements, such as scandium (Sc), yttrium (Y), lanthanum (La),cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm),gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium(Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Examples of themetal compound include oxide, hydroxide, nitride, oxynitride, fluoride,and chloride.

A portion of the calcium compound in the raw material mixture may bereplaced with a strontium compound, metal strontium, or an alloy ofstrontium as necessary. Examples of the strontium compound includestrontium-containing hydride, oxide, hydroxide, nitride, oxynitride,fluoride, and chloride.

The strontium compound may be appropriately selected from commerciallyavailable products, or a desired strontium compound may be produced. Forinstance, strontium nitride may be produced in the same manner ascalcium nitride. Unlike calcium nitride, strontium nitride is likely totake any amount of nitrogen, and is represented by SrN_(x). In theformula, x is, for instance, from 0.5 to 1.

In a raw material mixture containing a strontium atom, the ratio of thenumber of strontium atom in the total amount of calcium atom andstrontium atom in the raw material mixture may be, for instance, from0.1% by mol to 99.9% by mol, and preferably from 0.1% by mol to 98% bymol. When the content of the strontium atom is in this range, a desiredvalue of peak emission wavelength of the nitride fluorescent materialmay be achieved.

The mixing ratio of silicon nitride, silicon, an aluminium compound, acalcium compound, and a europium compound in the raw material mixture isnot particularly limited as long as the nitride fluorescent materialhaving a composition represented by formula (I) is obtained, and may beappropriately selected according to a desired composition. For instance,the molar ratio of silicon atom to aluminium atom in the raw materialmixture is u:v, and may be from 0.9:1.1 to 1.1:0.9. The molar ratio ofcalcium atom (optionally strontium atom is contained) to aluminium atomis (s+t):u, and may be 0.9:1 to 1.11:1. The molar ratio of the europiumatom in the total molar amount of calcium atom (optionally strontiumatom is contained) and europium atom may be, for instance, 1:0.05 to1:0.001, or 1:0.03 to 1:0.003.

For instance, mixing calcium nitride, europium oxide, aluminium nitride,silicon nitride, and silicon to prepare a raw material mixture having acomposition ratio of Ca:Eu:Al:Si=0.993:0.007:1:1, and heat-treating theraw material mixture by a method described later can produce a nitridefluorescent material represented by:

Ca_(0.993)Eu_(0.007)AlSiN₃.

The composition of the nitride fluorescent material, however, is arepresentative composition estimated by the mixing ratio of the rawmaterial mixture. Since europium oxide is used and each raw materialcontains about 1% by weight of oxygen, the actual fluorescent materialto be obtained may contain a certain amount of oxygen. To show arepresentative composition, the chemical formula given above does notcontain such oxygen. The resulting composition may slightly differ fromthe initial composition because a portion of the raw materials maydegrade and scatter out while undergoing heat treatment. By changing themixing ratio of each raw material, however, the composition of thetarget nitride fluorescent material may be changed. Although anexplanation is made here using a composition containing no strontium,needless to say, the same can be said about a composition containingstrontium.

The raw material mixture may further contain a separately preparedcomposition (nitride fluorescent material) represented by formula (I) asnecessary. When a raw material mixture contains a nitride fluorescentmaterial, the content may be, for instance, from 1% by weight to 50% byweight in the total amount of the raw material mixture.

The raw material mixture may contain a flux such as a halide asnecessary. In a raw material containing a flux, the reaction between rawmaterials may be further accelerated, and the solid phase reaction mayproceed more uniformly, so that a fluorescent material having a largeparticle diameter and superior light emitting properties can beobtained. This is, for instance, believed to be due to the fact that thetemperature of heat treatment in the preparation step is equal to orgreater than the temperature for generating a liquid phase such ashalide, which is a flux. Examples of the halide include chlorides andfluorides of rare earth metals, alkali earth metals, and alkali metals.The flux may be added as a compound that helps the element ratio of thecations to achieve the target composition, or may be further added as anadditive after various materials are added to make up the targetcomposition. When the raw material mixture contains a flux, an amount ofa flux may be, for instance, 20% by weight or less, or 10% by weight orless. The content is also, for instance, 0.1% by weight or more. A fluxin such an amount can accelerate reaction without lowering the emittingluminance of the fluorescent material.

The raw material mixture may be obtained by weighing desired materialcompounds in a desired compounding ratio, and then mixing the materialcompounds by a mixing method using a ball mill, or a mixing method usinga mixing machine, such as a Henschel mixer or a V-blender, or using amortar and a pestle. The mixing may be dry mixing or wet mixing byadding, for instance, a solvent.

The temperature of the heat treatment of the raw material mixture is,for instance, 1200° C. or more, 1500° C. or more, or 1900° C. or more.The heat-treating temperature is also, for instance, 2200° C. or less,2100° C. or less, or 2050° C. or less. Heat treatment at a temperatureof 1200° C. or more may allow easier incorporation of Eu into thecrystal, and efficient formation of a desired nitride fluorescentmaterial. Heat treatment at a temperature of 2200° C. or less is likelyto suppress degradation of the resulting nitride fluorescent material.

The atmosphere in the heat treatment of the raw material mixture is, forinstance, a nitrogen gas-containing atmosphere, or substantially anitrogen gas atmosphere. A nitrogen gas-containing atmosphere may allownitriding of silicon contained in the raw material. A nitrogengas-containing atmosphere also suppresses degradation of nitride, whichis a raw material, and the fluorescent material. When the atmosphere inthe heat treatment of the raw material mixture contains a nitrogen gas,the atmosphere may contain another gas, such as hydrogen, rare gas suchas argon, carbon dioxide, carbon monoxide, oxygen, or ammonia, inaddition to nitrogen gas. The content of the nitrogen gas in the heattreating atmosphere of the raw material mixture may be, for instance,90% by volume or more, or 95% by volume or more. When the content ofgases containing elements other than nitrogen is equal to or less than apredetermined value, the possibility for these gaseous components toform impurities and lower the light emitting luminance of fluorescentmaterial is further reduced.

The pressure in the heat treatment of the raw material mixture may be,for instance, from ordinary pressure to 200 MPa. To suppress degradationof the nitride fluorescent material to be generated, the pressure may behigh, for instance, from 0.1 MPa to 200 MPa, or from 0.6 MPa to 1.2 MPafor less restrictions on industrial equipment.

Heat treatment of the raw material mixture may be performed at a singletemperature, or by multiple stages including two or more heat-treatingtemperatures. Heat treatment by multiple stages, for instance, includesperforming a first-stage heat treatment at 800° C. to 1400° C.,elevating the temperature slowly, and then performing a second-stageheat treatment at 1500° C. to 2100° C.

In the heat treatment of the raw material mixture, the heat treatment isperformed, for instance, by elevating the temperature from roomtemperature to a predetermined temperature. The time for elevating thetemperature may be, for instance, from 1 hour to 48 hours, from 2 hoursto 24 hours, or from 3 hours to 20 hours. When the time for elevatingthe temperature is 1 hour or more, the fluorescent material particlesare likely to fully grow, and Eu is likely to be easily incorporatedinto the crystals of the fluorescent material particles.

In the heat treatment of the raw material mixture, a retention time at apredetermined temperature may be provided. The retention time may be,for instance, from 0.5 hour to 48 hours, from 1 hour to 30 hours, orfrom 2 hours to 20 hours. With a retention time of a predetermined valueor more, more uniform particle growth may be further accelerated. With aretention time of a predetermined value or less, degradation of thefluorescent material may be further suppressed.

The time for lowering the temperature from a predetermined temperatureto room temperature in the heat treatment of the raw material mixturemay be, for instance, from 0.1 hour to 20 hours, from 1 hour to 15hours, or from 3 hours to 12 hours. A retention time may be provided atan appropriately selected temperature while the temperature is loweredfrom a predetermined temperature to room temperature. This retentiontime may be, for instance, adjusted to further enhance thelight-emitting luminance of the nitride fluorescent material. Theretention time at a predetermined temperature while the temperature islowered may be, for instance, from 0.1 hour to 20 hours, or from 1 hourto 10 hours. The temperature during the retention time may be, forinstance, from 1000° C. to less than 1800° C., or from 1200° C. to 1700°C.

The raw material mixture may be heat-treated, for instance, in a gaspressurized electric furnace. Heat treatment of the raw material mixturemay be performed, for instance, by charging the raw material mixtureinto a crucible or boat made of a carbon material, such as black lead,or of a boron nitride (BN) material. Besides carbon materials or boronnitride materials, alumina (Al₂O₃) or Mo materials, for example, may beused. A crucible or boat made of boron nitride is particularlypreferable.

After the heat treatment of the raw material mixture, a sizing stepincluding operations in combination of crushing, grinding, andclassifying a nitride fluorescent material resulting from the heattreatment, may be performed. A powder of a desired particle diameter canbe obtained by the sizing step. Specifically, after roughly grinding anitride fluorescent material, the roughly ground particles may be groundusing a common grinder, such as a ball mill, a jet mill, and a vibrationmill to have a predetermined particle diameter. Excessive grinding,however, may cause defects on the surface of the fluorescent materialparticles, resulting in luminance decrease. When particles havingdifferent particle diameters are present after grinding, the particlediameters may be made uniform by classifying the particles.

[Nitride Fluorescent Material]

The present disclosure encompasses a nitride fluorescent materialproduced by the above-described production method. The nitridefluorescent material may contain an alkali earth metal, aluminium,silicon, and europium, or the nitride fluorescent material may have acomposition represented by formula (I). As a result of the raw materialmixture containing silicon and silicon nitride in combination, sinteringof the nitride fluorescent material in the heat treatment during theproduction may be suppressed, achieving a larger particle diameter andthus higher luminance.

The nitride fluorescent material is, for instance, a red-light emittingfluorescent material that absorbs light in the range of from 200 nm to600 nm, and emits light having a peak emission wavelength in the rangeof from 605 nm to 670 nm. The excitation wavelength of the nitridefluorescent material may fall within the range of from 420 nm to 470 nm.The half bandwidth in the light emission spectrum of the nitridefluorescent material may be, for instance, from 70 nm to 95 nm.

The specific surface area of the nitride fluorescent material may be,for instance, less than 0.3 m²/g, 0.27 m²/g or less, 0.2 m²/g or less,0.16 m²/g or less, 0.15 m²/g or less, or 0.13 m²/g or less. The specificsurface area may be also, for instance, 0.05 m²/g or more, or 0.1 m²/gor more. When the specific surface area is less than 0.3 m²/g, thenitride fluorescent material is likely to have further improved lightabsorption and conversion efficiency, achieving higher luminance.

The specific surface area of the nitride fluorescent material ismeasured by the Brunauer-Emmett-Teller (BET) method. Specifically, thespecific surface area is calculated by the dynamic constant-pressuremethod using Gemini 2370 manufactured by Shimadzu Corporation.

The average particle diameter of the nitride fluorescent material maybe, for instance, 15 μm or more, 18 μm or more, or 20 μm or more. Theaverage particle diameter may be, for instance, 30 μm or less, or 25 μmor less. When the average particle diameter is 15 μm or more, lightabsorption and conversion efficiency are likely to be further improved,achieving still higher luminance. When the average particle diameter is30 μm or less, it is likely that the nitride fluorescent material ishandled more easily, and the productivity of a light emitting devicecontaining the nitride fluorescent material is likely to be furtherimproved. The average particle diameter of the nitride fluorescentmaterial may be, for instance, in the range of from 15 μm to 30 μm.Preferably, the fluorescent material contains particles of this meanparticle diameter at high frequency. The particle sizes are alsopreferably distributed in a narrow range. A light emitting deviceincluding a nitride fluorescent material with less variation in particlediameter and particle size distribution has less color unevenness, andthus has favorable color tone.

The average particle diameter of the nitride fluorescent material isFisher Sub Sieve Sizer's No. (F.S.S.S.N.) obtained by the airpermeability method using Fisher Sub Sieve Sizer. Specifically, this isa value obtained by measuring each sample of 1 cm³ under the conditionsof an atmospheric temperature of 25° C. and a humidity of 70% RH,packing each sample into a dedicated tubular container, to which dry airat a given pressure is introduced, and a specific surface area is readfrom the difference in pressure and converted to an average particlediameter.

To improve emitting luminance, the nitride fluorescent material may havea specific surface area of less than 0.3 m²/g by the BET method and anaverage particle diameter of 18 μm or more, or a specific surface areaof 0.2 m²/g or less and an average particle diameter of 20 μm or more,or a specific surface area of 0.16 m²/g or less and an average particlediameter of 20 μm or more. The nitride fluorescent material may have aspecific surface area of 0.1 m²/g or more, and an average particlediameter of 30 μm or less, or 25 μm or less.

To improve emitting luminance, the nitride fluorescent material is anitride containing an alkali earth metal, aluminium, silicon, andeuropium, and may have a specific surface area by the BET method of 0.1m²/g to 0.16 m²/g and an average particle diameter of from 20 μm to 30μm. The nitride fluorescent material may have a composition representedby formula (I) and a specific surface area by the BET method of from 0.1m²/g to 0.16 m²/g and an average particle diameter of from 20 μm to 30μm. To improve emitting luminance, the nitride fluorescent material is anitride containing an alkali earth metal, aluminium, silicon, andeuropium, and may have a specific surface area by the BET method of from0.1 m²/g to 0.15 m²/g and an average particle diameter of from 20 μm to30 μm. Or the nitride fluorescent material may have a composition offormula (I) wherein s=0, a specific surface area by the BET method offrom 0.1 m²/g to 0.15 m²/g, and an average particle diameter of from 20μm to 30 μm.

The nitride fluorescent material may have a high crystalline structurein at least a portion thereof. For instance, since a glass body(amorphous) is of an irregular and less crystalline structure, thereaction conditions in the production process of a fluorescent materialmust be controlled to be strictly uniform. Otherwise, the ratio ofcomponents in the resultant fluorescent material varies, which is likelyto cause chromaticity unevenness, for example. In contrast, the nitridefluorescent material according to the present embodiment is a powder ora granule having high crystallinity in at least a portion thereof, andthus may be easily produced and processed. In addition, the nitridefluorescent material can be uniformly dispersed in an organic medium, sothat light emitting plastics and polymeric thin film materials, forexample, can be readily produced. Specifically, the nitride fluorescentmaterial may be a crystalline structure in, for instance, 50% by weightor more, or 80% by weight or more. This indicates the proportion of acrystalline phase having light emission properties, and the presence ofa crystalline phase of 50% by weight or more may ensure emission oflight enough for practical use. Thus the emitting luminance and ease ofprocessability increase with the proportion of the crystalline phase.

[Light Emitting Device]

The present disclosure encompasses a light emitting device incorporatingthe nitride fluorescent material. The light emitting device at leastincludes a light emitting element having a peak emission wavelength inthe range of, for instance, from 380 nm to 470 nm; and a fluorescentmember containing at least a first fluorescent material including thenitride fluorescent material. The fluorescent member may further containa second fluorescent material that emits green to yellow light. Thelight emitted by the light emitting device is a mix of light from thelight emitting element and fluorescence from the fluorescent member. Forinstance, the light may have chromaticity coordinates defined by CIE1931 of x=0.220 to 0.340 and y=0.160 to 0.340, or x=0.220 to 0.330 andy=0.170 to 0.330.

An example of a light emitting device 100 according to an embodiment ofthe present disclosure will be described with reference to the drawings.FIG. 1 is a schematic sectional view of an example of a light emittingdevice 100 according to the present disclosure. A light emitting device100 is an example of a surface mounting type light emitting device.

The light emitting device 100 includes a nitride gallium compoundsemiconductor-light emitting element 10 that emits visible light atshort wavelengths (e.g., in the range of from 380 nm to 485 nm) and hasa peak emission wavelength of, for instance, from 440 nm to 460 nm, anda molded body 40 on which the light emitting element 10 is disposed. Themolded body 40 includes a first lead 20, a second lead 30, and a resinportion 42, which are formed in an integral manner. Alternatively, amolded body 40 may be formed by a known method using a ceramic as amaterial in place of the resin portion 42. The molded body 40 has arecess defined by a bottom surface and side surfaces, and the lightemitting element 10 is disposed on the bottom surface of the recess. Thelight emitting element 10 has a pair of electrodes, positive andnegative, and the positive and negative electrodes are electricallyconnected to the first lead 20 and the second lead 30, respectively,with a wire 60. The light emitting element 10 is covered with afluorescence member 50. The fluorescence member 50 contains, forexample, a red light fluorescent material (a first fluorescent material71) and a green light fluorescent material (a second fluorescentmaterial 72) as a fluoride fluorescent material 70 that converts thewavelength of light emitted from the light emitting element 10, and aresin.

The fluorescence member 50 serves not only as a member containing afluorescent material 70 for converting the wavelength, but also as amember for protecting the light emitting element 10 and the fluorescentmaterial 70 from the outside environment. In FIG. 1, the particles ofthe fluorescent material 70 are unevenly dispersed in the fluorescencemember 50. Arranging the particles of the fluorescent material 70 closerto the light emitting element 10 in this manner allows efficientconversion of the wavelength of light from the light emitting element10, thereby providing a light emitting device with superior emittingefficiency. It should be noted that the arrangement of the particles ofthe fluorescent material 70 and the light emitting element 10 in thefluorescence member 50 is not limited to one in which they are in closeproximity to each other, and the particles of the fluorescent material70 may be arranged spaced apart from the light emitting element 10within the fluorescence member 50 to avoid the influence of heat on thefluorescent material 70. The particles of the fluorescent material 70may also be approximately evenly dispersed in the entire fluorescencemember 50 so as to obtain light with further reduced color unevenness.

(Light Emitting Element)

The peak emission wavelength of the light emitting element lies in therange of, for instance, from 380 nm to 470 nm, or from 440 nm to 460 nm.Using a light emitting element having a peak emission wavelength in thisrange as an excitation light source yields a light emitting device thatemits light resulting from a mix of the light from the light emittingelement and fluorescence from the fluorescent materials. In addition,because this allows effective use of light radiated from the lightemitting element to the outside, the loss of light emitted from thelight emitting device can be minimized, resulting in a highly efficientlight emitting device.

The half bandwidth of the light emission spectrum of the light emittingelement may be, for instance, 30 nm or less.

The light emitting element may be a semiconductor light emittingelement. Using a semiconductor light emitting element as the excitationlight source yields a highly efficient light emitting device that hashigh output linearity to the input and is resistant and stable tomechanical impact.

For example, as a semiconductor light emitting element, a nitridesemiconductor (In_(X)Al_(Y)Ga_(1-X-Y)N, wherein 0≤X, 0≤Y, and X+Y≤1)that emits blue or green light, for example, may be used as thesemiconductor light emitting element.

(Fluorescent Member)

The light emitting device includes a fluorescent member that converts awavelength by absorbing a portion of light emitted from a light emittingelement. The fluorescent member contains at least one first fluorescentmaterial that emits red light, and may contain at least one secondfluorescent material that emits green to yellow light. Examples of thefirst fluorescent material include the above-described nitridefluorescent material. For a second fluorescent material, a fluorescentmaterial may be appropriately selected from green light fluorescentmaterials that emit fluorescence having a peak emission wavelength inthe range of from 500 nm to 580 nm. By appropriately selecting the peakemission wavelength, light emission spectrum, and so forth of a secondfluorescent material, the properties such as correlated colortemperature and color rendering properties of the light emitting devicemay fall within the desired ranges. The fluorescent member may containresin in addition to fluorescent materials. A light emitting device mayinclude a fluorescent member that contains a fluorescent material and aresin, and covers a light emitting element.

The details of the nitride fluorescent material contained in the firstfluorescent material are as follows. The content of a first fluorescentmaterial in a light emitting device may be, for instance, from 0.1 partby weight to 50 parts by weight, or from 1 parts by weight to 30 partsby weight relative to 100 parts by weight of the resin contained in thefluorescent member.

The second fluorescent material emits fluorescence having peak emissionwavelength in the range of, for instance, from 500 nm to 580 nm, or from520 nm to 550 nm. The second fluorescent material may be one selectedfrom the group consisting of a β sialon fluorescent material having acomposition represented by formula (IIa); a silicate fluorescentmaterial having a composition represented by formula (IIb); ahalosilicate fluorescent material having a composition represented byformula (IIc); a thiogallate fluorescent material having a compositionrepresented by formula (IId); a rare earth aluminate fluorescentmaterial having a composition represented by formula (IIe); an alkalineearth aluminate fluorescent material represented by formula (IIf); andan alkaline earth phosphate fluorescent material represented by formula(IIg). In particular, containing, as a second fluorescent material, atleast one fluorescent material having a composition represented byformula (IIc), (IIe), (IIf) or (IIg) together with a first fluorescentmaterial in a fluorescent member is preferable, because this combinationimproves color rendering properties of the light emitting device.

Si_(6-w)Al_(w)O_(w)N_(8-w):Eu  (IIa)

wherein w satisfies 0<w≤4.2.

(Ba,Sr,Ca,Mg)₂SiO₄:Eu  (IIB)

(Ca,Sr,Ba)₈MgSi₄O₁₆(F,Cl,Br)₂:Eu  (IIc)

(Ba,Sr,Ca)Ga₂S₄:Eu  (IId)

(Y,Lu,Gd)₃(Al,Ga)₅O₁₂:Ce  (IIe)

(Sr,Ca,Ba)₄Al₁₄O₂₅:Eu  (IIf)

(Ca,Sr,Ba)₅(PO₄)₃(Cl,Br):Eu  (IIg)

In the formula (IIa), w may satisfy 0.01<w<2.

To improve emitting luminance, the average particle diameter of thesecond fluorescent material contained in a light emitting device ispreferably from 2 μm to 35 μm, or from 5 μm to 30 μm.

The average particle diameter of a second fluorescent material ismeasured in the same manner as the average particle diameter of a firstfluorescent material.

The content of a second fluorescent material in a light emitting devicemay be, for instance, from 1 part by weight to 70 parts by weight, orfrom 2 parts by weight to 50 parts by weight relative to 100 parts byweight of the resin contained in the fluorescent member.

The ratio of a first fluorescent material to a second fluorescentmaterial (first fluorescent material/second fluorescent material) in alight emitting device is, for instance, from 0.01 to 10, or from 0.1 to1 in terms of weight.

Other Fluorescent Materials

The light emitting device may contain another fluorescent material asnecessary in addition to a first fluorescent material and a secondfluorescent material. Examples of the other fluorescent material includeCa₃Sc₂Si₃O₁₂:Ce, CaSc₂O₄:Ce, (La,Y)₃Si₆N₁₁:Ce, (Ca,Sr,Ba)₃Si₆O₉N₄:Eu,(Ca,Sr,Ba)₃Si₆O₁₂N₂:Eu, (Ba,Sr,Ca)Si₂O₂N₂:Eu, (Ca,Sr,Ba)₂Si₅N₈:Eu, andK₂ (Si,Ti,Ge)F₆:Mn. When the light emitting device contains otherfluorescent materials, the content thereof is, for instance, 10% byweight or less, or 1% by weight or less relative to the total amount ofa first fluorescent material and a second fluorescent material.

The resin in the fluorescent member may include a thermoplastic resin ora thermosetting resin. Specific examples of the thermosetting resininclude an epoxy resins and a silicone resin. The fluorescent member maycontain another component as necessary in addition to a fluorescentmaterial and a resin. Examples of the other component include a fillersuch as silica, barium titanate, titanium oxide, and aluminium oxide; alight stabilizer; and a colorant. When the fluorescence member containsanother component, for instance, a filler, the amount contained may befrom 0.01 part by weight to 20 parts by weight relative to 100 parts byweight of the resin.

EXAMPLES

Hereinafter, the Examples of the present disclosure will be specificallydescribed, but the present disclosure is by no means limited to theseExamples.

Example 1

Calcium nitride (Ca₃N₂), silicon nitride (Si₃N₄), elemental silicon(Si), aluminium nitride (AlN), and europium oxide (Eu₂O₃), whichconstitute a raw material compound, were weighed so as to have a molarratio of Ca:Si:Al:Eu=0.993:1.1:0.9:0.007, and mixed. Here, siliconnitride and elemental silicon were mixed so that the silicon nitrideconstitutes 41.6% by weight and elemental silicon constitutes 58.4% byweight. The resultant mixed raw material was charged into a cruciblemade of boron nitride, and heat-treated in a nitrogen atmosphere underthe pressure of 0.92 MPa (gauge pressure) at 2000° C. for 2 hours toobtain a nitride fluorescent material.

Example 2

A nitride fluorescent material was obtained in the same manner asExample 1 except that the mixing ratio of silicon nitride to elementalsilicon was changed so that silicon nitride constitutes 37.5% by weightand elemental silicon constitutes 62.5% by weight.

Example 3

A nitride fluorescent material was obtained in the same manner asExample 1 except that the mixing ratio of silicon nitride to elementalsilicon was changed so that silicon nitride constitutes 20.5% by weightand elemental silicon constitutes 79.5% by weight.

Example 4

A nitride fluorescent material was obtained in the same manner asExample 1 except that the mixing ratio of silicon nitride to elementalsilicon was changed so that silicon nitride constitutes 70.6% by weightand elemental silicon constitutes 29.4% by weight.

Example 5

A nitride fluorescent material was obtained in the same manner asExample 1 except that the mixing ratio of silicon nitride to elementalsilicon was changed so that silicon nitride constitutes 84.4% by weightand elemental silicon constitutes 15.6% by weight.

Comparative Example 1

A nitride fluorescent material was obtained in the same manner asExample 1 except that silicon nitride alone was used without usingelemental silicon.

Comparative Example 2

A nitride fluorescent material was obtained in the same manner asExample 1 except that elemental silicon alone was used without usingsilicon nitride.

The resultant nitride fluorescent materials were evaluated for thefollowing points.

Average Particle Diameter:

Samples of 1 cm³ were weighed using a Fisher Sub Sieve Sizer (F.S.S.S.)under the conditions of a temperature of 25° C. and a humidity of 70%RH. Each sample was packed into a dedicated tubular container, intowhich dried air of a given pressure is flown. For each sample, thespecific surface area is determined from the difference in pressure, andthe average particle diameter was calculated.

Specific Surface Area

For each nitride fluorescent material, the specific surface area wascalculated by the dynamic constant-pressure method using Gemini 2370manufactured by Shimadzu Corporation in accordance with the instructionmanual.

Light Emitting Properties

The light emission spectrum of each nitride fluorescent material whenexcited at 460 nm was measured using F-4500 manufactured by HitachiHigh-Technologies Corporation. The energy value: ENG (%) and peakemission wavelength: λp (nm) of the resultant light emission spectrumwere calculated.

Table 1 shows the average particle diameter, specific surface area, λp,ENG (%) of each nitride fluorescent material. Each ENG (%) is a relativevalue when the energy value of the nitride fluorescent material ofComparative Example 1 is taken as 100%. FIG. 2 shows light emissionspectra.

TABLE 1 Weight Average Specific Light percentages particle surfaceemitting of Si material (%) diameter area properties Si Si₃N₄ (μm)(m²/g) λp (nm) ENG (%) Example 1 58.4 41.6 23.0 0.11 646 118.9 Example 237.5 62.5 23.0 0.13 651 115.4 Example 3 20.5 79.5 19.5 0.16 649 104.0Example 4 70.6 29.4 20.5 0.11 649 114.9 Example 5 84.4 15.6 23.0 0.27653 102.3 Comparative 0 100 17.0 0.23 652 100.0 Example 1 Comparative100 0 27.5 0.30 653 98.1 Example 2

Comparative Example 1 is a fluorescent material using no elementalsilicon and calcinated at 2000° C., and the ENG value of ComparativeExample 1 serves as the reference. In Examples 1 to 5 where siliconnitride and elemental silicon were used in combination, specific surfaceareas were each less than 0.3 m²/g, the average particle diameters wereeach 18 μm or more, and ENGs were high, so that light emittingproperties were superior.

FIGS. 3 and 4 show scanning electron microscope (SEM) images of thenitride fluorescent materials of Comparative Example 1 and Example 1. InComparative Example 1 shown in FIG. 3, large particles and fineparticles are mixed. This is believed to be due to the fact thatparticles are sintered together when calcinated at a high temperature,and these particles are ground together during grinding process fordispersion to yield finer microparticles. In Example 1 as shown in FIG.4, no such microparticles are present, because less grinding ofparticles occurred when the calcinated products were ground due to lesssintering. The particles of the nitride fluorescent material of Example1 suffer little damage during grinding process, so that the surfaces ofparticles are smooth as shown in FIG. 4, achieving high ENG because ofno mixture of microparticles with lower emitting luminance. A lightemitting device containing a nitride fluorescent material of the presentExamples are also believed to contain little microparticles that causesRayleigh scattering, so that scattering of light emitted from the lightemitting element towards the inside of the light emitting device (lightemitting element) is minimized, accelerating scattering of light towardsthe outside of the light emitting device, i.e., the surface for takingout light (e.g., Mie scattering), achieving a light emitting devicehaving high emitting efficiency.

This is believed to be attributable to the fact that, for instance, byusing silicon nitride and elemental silicon in combination as rawmaterials, the amount of oxygen is reduced and thus sintering issuppressed compared to the case where silicon nitride alone is used, andchange in volume that occurs when silicon turns into silicon nitride isalso utilized to promote particle growth and reduce susceptibility tosintering.

In contrast, in Comparative Example 2 where no silicon nitride is used,the particle diameter and specific surface area are larger and ENG islower. Since the formation of the fluorescent material and the nitridingof silicon are performed simultaneously, silicon is believed to benitrided insufficiently, resulting in poor properties. Using siliconnitride and elemental silicon in combination accelerates nitriding ofsilicon, which is believed to produce this difference.

Example 6

A nitride fluorescent material was obtained in the same manner asExample 1 except that strontium nitride was used as a strontiumcompound, that the composition of the raw material mixture was changedso as to have a molar ratio of Sr:Ca:Si:Al:Eu=0.099:0.891:1.1:0.9:0.01,and that the ratio of silicon nitride to elemental silicon was 37.5% byweight to 62.5% by weight.

Comparative Example 3

A nitride fluorescent material was obtained in the same manner asExample 6 except that silicon nitride alone without elemental siliconwas used.

The resultant nitride fluorescent materials were evaluated in the samemanner as described above. Table 2 shows the average particle diameter,specific surface area, λp, and ENG (%) of each nitride fluorescentmaterial. ENG (%) is a relative value when the energy value of thenitride fluorescent material of Comparative Example 1 is taken as 100%.FIG. 5 shows resultant light emission spectra.

TABLE 2 Light Weight percentages Average Specific emitting of Siparticle surface properties material (%) diameter area λp ENG Si Si₃N₄(μm) (m²/g) (nm) (%) Example 6 37.5 62.5 22.0 0.16 657 111.3 Comparative0.0 100.0 19.5 0.31 663 99.5 Example 3

As shown in Table 2 and FIG. 5, Example 6 has a peak emission wavelengthof 657 nm, and Comparative Example 3 has a peak emission wavelength of663 nm, both of which are longer than that of Example 1. This isbelieved to reflect changes in the amount of Eu. Like Examples 1 to 5,Example 6 also has a specific surface area of as small as 0.2 m²/g orless, as a result of elemental silicon being added to the raw materials,and has higher light emitting properties than Comparative Example 3,demonstrating good results.

A light emitting device including a nitride fluorescent materialobtained by a method for producing according to the present embodimentmay be suitably used as, for example, a light source for lighting. Inparticular, the light emitting device may be suitably used for a lightsource for lighting, LED displays, backlight light sources, trafficsignals, illuminated switches, and various indicators. Since a nitridefluorescent material having a high emitting luminance, for example, isobtained, the industrial applicability is significantly high.

It is to be understood that although the present invention has beendescribed with regard to preferred embodiments thereof, various otherembodiments and variants may occur to those skilled in the art, whichare within the scope and spirit of the invention, and such otherembodiments and variants are intended to be covered by the followingclaims.

All publications, patent applications, and technical standards mentionedin this specification are herein incorporated by reference to the sameextent as if each individual publication, patent application, ortechnical standard was specifically and individually indicated to beincorporated by reference.

What is claimed is:
 1. A nitride fluorescent material comprising analkali earth metal, aluminium, silicon, and europium, and having aspecific surface area by the BET method of from 0.1 cm²/g to 0.16 cm²/gand an average particle diameter of from 20 μm to 30 μm.
 2. The nitridefluorescent material according to claim 1, wherein the nitridefluorescent material has a specific surface area by BET method of from0.1 cm²/g to 0.15 cm²/g.
 3. The nitride fluorescent material accordingto claim 1, wherein the nitride fluorescent material has a compositionrepresented by formula (I):Sr_(s)Ca_(t)Al_(u)Si_(v)N_(w):Eu  (I) wherein s, t, u, v, and wrespectively satisfy 0≤s<1, 0<t≤1, s+t≤1, 0.9≤u≤1.1, 0.9≤v≤1.1, and2.5≤w≤3.5.
 4. The nitride fluorescent material according to claim 3,wherein s=0 in formula (I).